Recent developments in the environmental and logistical factors governing the supply and consumption of available conventional energy resources have highlighted the urgent need to search for and exploit alternate sources of energy. When one selects a given energy source, he is inevitably faced with a number of difficult problems. Traditionally, the principle sources of energy have been the fossil fuels, such as coal and petroleum. A century ago, these fuels were found in great abundance on earth, but the rapid pace of industrialization, especially in the western economies, has resulted in the rapid consumption of these resources and an unanticipated shortage in their supply.
Today, remaining world reserves of petroleum are rapidly approaching extinction, and even those sources which are producing at relatively high levels have, even now, proven themselves, for reasons not related to availability, to be unreliable. Quite apart from the quantity of remaining fossil fuel and the availability of that fuel, one must also consider the environmental pollution generated in the conversion of fossil fuels into useful energy forms. This problem is particularly acute in the case of coal which often includes large quantities of sulphur. Sulphur compounds enter the air and pollute the atmosphere during combustion of the sulphur-containing fuels. Even nuclear energy, which should be able to supply our energy needs into the visible future, could cause serious pollution problems and, indeed, if the nuclear waste products already produced as a result of past nuclear reactions should be released, the result would be an environmental catastrophe. Thus, even if we were to assume unlimited supplies of energy, increasing industrial development would result in a threat or damage to the environment.
In an attempt to overcome the deficiencies of fossil fuels discussed above, scientists have been led to consider a number of alternative energy sources. Insofar as the sun is ultimately the source of all energy on the earth, the direct use of sunlight appears to be one of the more promising solutions to present energy supply problems. Systems using sunlight as a source of power are already in widespread use throughout the world. For example, many buildings are architecturally designed to take maximum advantage of sunlight for the purpose of heating the building during the winter months, while minimizing the effect of the sun during the summer months. In addition to this, there are a number of commercially available systems which employ sunlight to heat water for use in a building.
Perhaps the most promising application of solar energy is in the generation of electrical power. Systems for doing this generally include an array of photovoltaic devices which are disposed over and completely cover a planar support surface. Photovoltaic devices offer a great deal of advantage in comparison to other solar energy converting devices. For example, the electricity generated may be conventionally sent from point to point over conventional electrical transmission lines. Electric energy is easily converted into other forms of energy and, when obtained from a photocell, is a non-polluting and inexhaustible energy source. These properties have made solar energy cells ideal energy sources in such applications as man-made orbital satellites and the like. However, solar cells suffer from a number of inadequacies which prevent their employment on a widespread scale. Perhaps the chief limiting factor is the limited area which is available for the collection of solar energy. In aerospace systems, this deficiency is compensated for by the use of large area solar panels which extend from the vehicle or satellite and are positioned to receive radiant energy. Even in terrestrial systems, the use of photocells is limited due to the inavailability of large areas for the collection of sunlight energy.
In connection with this, it is noted that the inadequate production of electricity from an array exposed to the sun is not due to an inadequate amount of energy falling on the energy converting cell or cells, but rather to the inefficiency with which conventional photoelectrical devices convert sunlight into electrical energy. Moreover, in addition to the inefficiencies involved in a photocell array, there is a tendency of the cells to saturate at a given level of incident radiation. Thus, as the level of light is increased from zero, the increasing light that is incident on a photocell results in greater electrical output until the saturation level is reached, at which point marginal increases in incident light result in insignificant or nonexistent increases in electrical power output. For most photocells, the threshold level of incident radiation is quite low relative to the intensity of bright sunlight. This problem is especially acute when one wishes to use reflectors or other systems to concentrate sunlight.